Medium tension lines carry electricity for distribution to end users. Such lines typically carry potential of 4 kilovolts to 33 kilovolts, measured phase to phase. They typically are configured as single phase and neutral, three phases without neutral, or three phases with neutral, although other combinations exist. The distribution line is often aerial until it gets close to homes or urban areas, where it often is converted to underground distribution. Underground medium tension cables are usually coaxial single phase cables. These cables have a coaxial, insulated shield that carries neutral current and protects against buildup of electric fields, which would be dangerous to the touch. Properly connected underground cables are safe to touch without insulated gloves or other protection.
Where the underground cables connect to the aerial cables, special care is taken at the junction. Aerial cables often are bare (not insulated) or covered only with weather-proof sheathing. Underground cables have multi-layer insulation between the phase conductor in the center and the coaxial neutral shield, which is covered by an insulating layer on the outside. To prevent an electric arc from running down the outside of the cable, a “stress cone” is connected to the end of the underground cable. The coaxial neutral shield is peeled back where it meets the stress cone and is connected to ground.
Medium tension lines emanate from a substation where there is a transformation from transmission voltage (typically 60-150 kilovolts) down to the appropriate medium tension voltage level. Trunk distribution lines radiate from the substation, and branch distribution lines emanate from the trunk lines.